It is said that the length of the column of clear plasma in a narrow tube left by erythrocytes which gradually sediments after one hour is the measure of ESR(erythrocyte sedimentation rate). Its normal values are found to be 3-5 mm after 1st hour for male and 4-7mm after 1st hour for female. What I want to know is why "1 hour" has been chosen. Why not 30min, 2hr, 3hr etc?
Does it really make a difference how long they settle, or why they chose that amount of time? At one extreme, you haven't really given them adequate time, at the other, there's no more useful information to gather.
The tubes in which Sed Rates (or ESR) are measured are 200 mm in height. Why wait three hours for the result (which is still expressed as a rate, i.e. mm/hr)? In three hours, they may (depending on how quickly they settle) have all settled an hour and a half ago. (If an ESR is 100mm/hr, it will hit the settled cells at the bottom before it finishes settling if two hours is the length of the test.
The test is simple, cheap, easy, with good sensitivity and specificity. And the results come back in a little over an hour. The sooner the diagnostician gets the results back, the sooner the answer helps in the decision tree needed for the treatment of the patient. Longer wait times aren't helpful and may not be accurate with very elevated ESRs.
Acute Phase Proteins Molecular Biology, Biochemistry, and Clinical Applications By Andrzej Mackiewicz, Irving Kushner, Heinz Baumann, 1993